Hot pressing is performed when an object product is formed or compression bonded in a process of manufacturing a precision equipment component having a laminated structure (referred to as the laminated board hereinafter) such as a printed circuit board such as a copper-clad laminated board, a flexible printed circuit board (FPC) and a multi-layer laminated board, an IC card, a ceramics laminated board, a liquid crystal display panel and the like.
According to the hot pressing, as shown in FIG. 4, a method in which laminated board material 17 which is an object to be pressed is interposed between hot platens 18 and a constant pressure and heat are applied thereto is used in general. In order to provide a laminated board with high precision, it is necessary to apply the heat and the pressure to the laminated board material 17 uniformly over an entire surface thereof. Thus, the hot pressing is performed using a cushion material 1 in the shape of flat plate interposed between the hot platen 18 and the laminated board material 17.
When the laminated board material 17 has fine irregularities of circuits on a surface such as an FPC or a multilayer board, or has relatively large irregularities such as a flex-build board which has a structure of integrated FPC and multilayer board, it is necessary for the cushion material 1 to follow the irregularities of the laminated board material 17 sufficiently. If the cushion material 1 does not sufficiently follow up the irregularities of the laminated board material 17, the heat and pressure cannot be uniformly applied to the entire surface of the laminated board material 17. As a result, a bonding defect is generated or air bubbles remain in the laminated board.
Japanese Unexamined Patent Publication No. 2000-52369 (patent document 1) discloses a hot-press cushion material in which fluoro rubber having high heat resistance and high durability is used. Particularly, a hot-press cushion material using a fluoro rubber having durometer hardness A60 is disclosed in the document in order to follow up the irregularities of the object to be pressed. However, since the printed circuit has become increasingly fine recently, the cushion material using the conventional fluoro rubber does not sufficiently follow up the irregularities. Therefore, in order to further improve following characteristics to the irregularities, the fluoro rubber having lower hardness has been required. However, the fluoro rubber has original characteristics in which it is not likely to be softened. When a method of lowering the hardness by compounding a lot of softener or plasticizer in the fluoro rubber is used, gas or bleeding is generated from the fluoro rubber at the time of the hot pressing. As a result, the pressing apparatus or the laminated board is contaminated, which causes a serious defect. Therefore, a low-molecular material such as the softener or plasticizer is not to be used as much as possible.
Meanwhile, as a method of lowering the hardness of the fluoro rubber, while preventing bleeding of the plasticizer, a method of compounding a liquid rubber such as a liquid fluoro rubber and the like in the fluoro rubber composition instead of the plasticizer is well-known in Japanese Unexamined Patent Publication No. 10-158458 (patent document 2), Japanese Unexamined Patent Publication No. 7-53821 (patent document 3) and Japanese Unexamined Patent Publication No. 6-116464 (patent document 4). However, in a case of the application of hot-press cushion material, the fluoro rubber compounding the liquid rubber is not preferable because it is expected that the liquid rubber bleeds at high temperature under high pressure.
In addition, a vulcanized fluoro rubber composition having low hardness of A50 or less is disclosed in Japanese Unexamined Patent Publication No. 4-268357 (patent document 5). However, since a raw fluoro rubber disclosed in the patent document 5 is characterized by intrinsic viscosity and molecular weight distribution, it can be provided only by particular synthesis composition. As a result, it is difficult to obtain it from a commercially available raw fluoro rubber, so that a manufacturer who forms or processes the rubber products cannot easily implement it.
Thus, it is very hard to provide the vulcanized fluoro rubber which is suitable for being used as the hot-press cushion material and has durometer hardness lower than A60.